This invention relates generally to machines to facilitate the mounting of flexographic printing plates and for obtaining proofs thereof, and more particularly to a mounting-proofing machine of relatively simple, low-cost design capable of handling plate cylinders in a large range of diameters.
In the flexographic process, printing is effected by rubber printing plates mounted on cylinders, the paper to be printed being impressed on the inked printing plate. The cylinder on which the printing plates are mounted is generally called the plate cylinder. The quality of a flexographic printing job depends, in large measure, on the care with which pre-press preparations are carried out. Plate-mounting, color registration and proofing are effected off the press by means of commercially available mounting-proofing machines designed for this purpose.
These machines, which usually make use of an optical mounting system, make it possible to mount the plates on plate cylinders to effect exact color-registration, a procedure essential to the maintenance of both quality and economy in all flexographic operations. Pre-proofing is, in many respects, the most important of all pre-press preparations, for it not only indicates the appearance of the final reproduction, but it also affords means to check the mounting of the plates for color sequence, spacing requirements, layout and gear size, as well as copy and color separation.
Mounting-proofing machines are provided with a proofing cylinder (sometimes called the impression cylinder) which cooperates with the plate cylinder, the proofing cylinder making contact with the printing plates on the plate cylinder and rotating concurrently therewith to print a proof on a sheet secured to the proofing cylinder. In commercial machines of the type which are presently available, the proofing or impression cylinder is supported for rotation at a fixed position, whereas the plate cylinder is movable in a vertical plane, from a mounting state in which it is retracted relative to the proofing cylinder to a proofing state in which it is in engagement therewith. One such machine is the M15 Mounter-Proofer manufactured by the Mosstype Corporation of Waldwick, New Jersey, the assignee of the present application.
The M15 machine includes a single element, line-of-sight viewer which shows the operator both the plate he is mounting on the plate cylinder and an image reflected from the proof sheet on the impression cylinder. By simply merging the two images, the operator sees exactly where to lay the plate on the plate cylinder. The entire viewer assembly automatically raises out of the way, giving unobstructed access to the plate cylinder for close inspection, inking, etc. The viewer returns automatically to the identical viewing position when ready to proceed with mounting.
The impression and plate cylinders are provided with respective gears of the same diameter. These gears are mechanically intercoupled, whereby rotation of the proofing cylinder causes the plate cylinder to rotate in both the mounting state when the cylinders are separated and in the proofing state when the cylinders are in engagement with each other.
When the diameter of the proofing cylinder is the same as the printing diameter of the plate cylinder (i.e., the diameter of the plate cylinder plus the thickness of the printing plates thereon), then a one-to-one relationship exists therebetween. In practice, however, the plate cylinders come in a range of diameters for different size printing jobs. This is a direct function of plate cylinder circumference which equals the pitch line circumference of the plate cylinder gear. It is necessary, therefore, in the M15 machine and in other mounting-proofing machines operating on similar principles to adjust the phase relationship between the plate and proofing cylinders to accommodate the differences between the cylinder diameters.
In order to mechanically interlink the plate cylinder and impression cylinder gears so that rotation of one cylinder results in rotation of the other regardless of their relative positions and for adjusting the phase relationship for different plate cylinder diameters, existing types of mounting-proofing machines require a relatively complex mechanism which adds substantially to the cost thereof.